The field of the present invention is needleless hypodermic injection methods and devices.
Various needleless hypodermic injection devices have been known and used in the past. These devices, also known as jet injectors, typically use spring or compressed gas driven plungers to accelerate an injectant to a velocity sufficient to pierce through the skin and enter the underlying tissues.
While large jet injection apparatus have been successfully used for mass inoculations, e.g. in the military services, these apparatus are relatively complex, costly, limited in performance and are not portable. Thus, injections using needles remain as the standard despite their disadvantages (for example, accidental needle sticks and risk of spreading infection to both the patient and medical professional; safe disposal of the used needle, patient's fear of needles; and pain caused by needle injections). Jet injection avoids or diminishes these disadvantages.
Although many portable needleless injectors have been proposed, these known devices have not achieved widespread acceptance in the medical field, due to a variety of factors.
Significantly, the characteristics of needleless or jet injections typically vary with the pressures exerted by the injection device, the nozzle diameter of the ampule, the patient's size, age and weight, the nature of the injection site, and the viscosity of the injectant.
The soft layers of tissue at standard injection sites in humans, listed in the order from outside to the inside are: 1) the dermis, 2) the adipose, 3) the deep fascia, a tough membrane that surrounds the muscle, and 4) the muscle. The deep fascia and the skin are the toughest layers to penetrate with a jet injection. The adipose tissue is the most easily penetrated.
Parenteral injections into humans are classified according to four well established tissue regions in which the injectant may be deposited. These are: intra-dermal, subcutaneous, intra-muscular, and intravenous. With intra-dermal injections, the injectant is deposited in the dermis layer. With subcutaneous (SC) injections, the injectant is deposited in the adipose tissue. With intramuscular injections (IM), the injectant is deposited in the muscle. Intra-venous are those injections deposited directly into a vein, an injection method generally not suitable for jet injection.
Intradermal injections, the least invasive of the three types, are employed when the dose is very small and it is desired to visualize patient response. Subcutaneous injections are employed when it is desired to prolong the time for absorption of the medication, when the dose is relatively small, or the injectant is non-irritating. Intramuscular injections, the most invasive of the three types, are employed when it is desired to have rapid absorption, when the medication is irritating, or when the dose is relatively large.
Absorption is not solely dependent on placement of the injectant, it is also dependent on the medication. Some medications are formulated to slow the rate of absorption. For example, intramuscular medications are sometimes oil based for this purpose. Similarly, subcutaneous medications sometimes contain crystalline compounds to delay absorption.
A long standing basic difficulty with jet injection has been the complex problem of determining which are the preferred injection variables. These variables include: 1) pressure profile, 2) nozzle size, 3) patient factors, i.e., age, sex and size, 4) injection site, and 5) medication viscosity. The repeated failures of the prior art to adequately solve these complex variables problems has contributed to the lack of acceptance of a handheld and portable jet injector in the medical community.
The pressure profile is the pressure exerted on the liquid injectant, typically measured over time, from the beginning to the end of the injection. The pressure profile must be selected, in combination with the nozzle size and other factors, to deliver the injectant through the skin to the desired depth, preferably with minimum pain.
The patient factors are also important. Gender is significant as women typically have a different adipose distribution than men. Men also typically have tougher tissue that women. The patient's age is important because infants are born with very little muscle, thick layers of adipose, and very easily penetrated skin. As infants age and become mobile the adipose is gradually replaced by muscle. At adolescence the introduction of hormones changes tissue composition. Aging through mid-life is usually associated with gradual weight gain and decrease in tissue strength.
Injection sites are very significant because in all patients the thickness of the skin and adipose tissue varies at different regions of the body. The medical profession has established generally accepted injection sites for conventional needle syringes that are best suited for specific types of injection. The subcutaneous sites typically have a thick adipose layer and are free of major nerves and vasculature. Intramuscular sites typically have a thin adipose layer, a thick muscle layer, and are free of major nerves and vasculature.
Finally, the viscosity of the injectant must be considered as it effects characteristics of the jet injection. In addition, it has been discovered that viscosity effects have been widely misunderstood in the prior art.
The prior art has generally not been able to overcome the complexities and difficulties of simultaneously accounting for all of the foregoing variables. Thus, jet injection, despite its great potential advantages, remains virtually unused. Accordingly, it is an object of the invention to provide improved methods and devices for needleless injection, so that the advantages of jet injection may be brought into use.